[Coco] Today, I have seen the CoCo 4...

[jdaggett at gate.net]

On 27 May 2010 at 22:52, Mark Marlette wrote:

> Frank,
> 
> I disagree. The FPGA is a BGA. As I recall the DE1 has 584 pins in about 1" sq. The hobbiest can't assemble this, let alone verify the quality of workmanship. You must have XRAY to verify the solder columns/balls.
> 
> A board house will want to set up a line or manually place the BGAs. I have such a machine at work that I run but we don't have a XRAY so we don't do BGAs, just QTFPs. Either way my experience has been ~100 boards just to get the line setup right. Either way is $$$$$.
> 
> I doubt it can be done in 4 layers, so 6 or 8. You will need almost all the resources that are on the DE1 and they produce those in several thousand boards runs, if I was to guess.
> 

584 ball BGA package will need 8 layers. Six will be pushing it. With 23 columns and 25 
rows, you need the layers to route out from under the package. DIgilent Inc Nexys2 board 
uses a 320 ball BGA and they use an 8 layer board for that product. 

BGAs in that large density is not in the realm of the home hobiest. up to about 80 pins and 
you start to run into issues for the hobbiest. Unfortunately QFPs are the only route the 
hobbiest has. 

> Putting on a larger SDRAM, etc. So how can one conclude that it could be built it cheaper? Hard to compete againest volume pricing.
> 

Even at volume pricing DRAM in general for equal density is and has always been cheaper 
than SRAM. That is in part that the DRAM memory bit requires less transistors than that of 
SRAM. SRAM cell is either 4 or 6 transistors depending on techniqic. DRAM can be done 
with as little as one transistor . DRAM will always be cheaper than SRAM. What makes 
SDRAM a bit more complicated is that it no longer is asynchronous. Instead there is a clock 
that access and refresh are all synched to. 



> Don't get me wrong, I would love to see it for $50 but then again, the FPGA costs more than that.
> 

A $50 dollar board is doable if a small FPGA is about all you want on it. What I invsision is 
closer to $200 - $250 range. XC3S500E-4 in a 208 pin QFP are ~$25 unit cost. That is not to 
bad for hobiest pricing. 

james